We have had a few mill puffs when the feeder speed is low.
We run 25% for minimum and move up quickly from there. Our burner flames are not stable (from the operator's point of view) without oil at that level.
2" Hg is big. The 30 foot height will not affect the pressure reading a measurable amount. Your are talking about a column of steam under a vacuum. You would need in the order of 20,000 ft to get that pressure difference.
Just a thought what do your turbine exhaust temperatures read...